Danny Martindale/WireImage(LONDON) -- Kate Middleton left King Edward VII Hospital in London Thursday morning after being admitted four days ago following the palace's announcement that she is pregnant and being treated for hyperemesis gravidarum.

"The Duchess of Cambridge has been discharged from the King Edward VII Hospital and will now head to Kensington Palace for a period of rest," Nick Loughran, the Assistant Press Secretary to the Duke and Duchess of Cambridge, said in a statement. "Their Royal Highnesses would like to thank the staff at the hospital for the care and treatment The Duchess has received."

Middleton, who is less than 12 weeks pregnant, was seen leaving the hospital with Prince William at 11 a.m. GT Thursday. A smiling Middleton was holding yellow flowers and waved to the crowd as she departed from the hospital in a black car.

The Duke and Duchess were spending time with her parents in Bucklebury when she became ill with the symptoms of hyperemesis gravidarum, or acute nausea.

Prince William sprung into action and drove his wife along with their personal security team 50 miles in their Range Rover to the hospital, where Kate was placed on an IV drip.

The royal family was only notified of Kate's pregnancy mere hours before the rest of the world.

A palace source told ABC News that the royal couple decided to go public with the pregnancy because Middleton had to be hospitalized Monday afternoon.

Hyperemesis gravidarum, or acute nausea, is usually diagnosed about nine weeks into a pregnancy, and in most cases resolves itself by 16 or 20 weeks, according to Dr. Ashley Roman, a professor and obstetrician-gynecologist at New York University Langone Medical Center. In rare cases, it can last the whole pregnancy.